Various types of projection type displays are currently used as image display devices for consumer TV, presentation, industry, large theater, and the like. In general, light is emitted to a spatial light modulating means called a light valve (optical valve) to form an image and then light passing therethrough or light reflected thereon is projected to an auxiliary surface such as a screen to display the image.
Up to now, this type of projection type display has a generally employed structure in which a lamp light source for the projection type display is replaced by a laser light source to make it possible to widen a color reproduction range obtained by a monochromatic spectrum, increase a contrast by light source intensity modulation, lengthen the life using a semiconductor laser or the like, and reduce a size of an optical system in view of light directivity, thereby obtaining performance which is significantly higher than that of the conventional lamp light source (see, for example, Patent Document 1). Note that an optical system for lighting the light valve with light emitted from a light source is called a lighting optical system and an optical system for enlarging and projecting a light beam emitted from the light valve is called a projection optical system.
When laser light is emitted to the light valve and projected to the screen, a light and dark spot pattern, which is called speckle, appears on an image. This is because, the laser light which is spatially and temporally coherent light is reflected on an optically rough surface or passes therethrough to become light having a different phase, so the light having the different phase is observed as interference light by human's eyes. The speckle becomes an image noise component and thus this is not preferable for an observer.
An example of a method of removing the speckle includes a method of coupling light from a laser light source to a multimode optical fiber and vibrating the optical fiber to cause a mode scramble (see, for example, Patent Document 2). According to this method, speckle patterns change. When the speckle patterns are superimposed on one another, speckles are averaged and reduced. However, a mechanical apparatus for vibrating the optical fiber is required. The durability of the mechanical apparatus and the optical fiber to be vibrated become a problem. The optical fiber having the circular cross sectional shape generates a skew component along the circumferential direction. Therefore, a spatial bias of the speckle pattern is large, so the speckles cannot be sufficiently removed.
On the other hand, another method of removing the speckle is a method of modulating an injection current of a semiconductor laser to set a multiwavelength oscillation mode for the semiconductor laser and performing spatial spectroscopy by an optical system using a diffraction grating (see, for example, Non-patent Document 1). Therefore, multiwavelength speckle patterns generate. When the speckle patterns are superimposed on one another, the speckles are averaged and reduced. However, the alignment adjustment of the optical system using the diffraction grating is complicated, so an apparatus becomes larger and expensive.
Another method of removing the speckle is a method of injecting a radio frequency signal to set the multiwavelength oscillation mode for a semiconductor laser (see, for example, Patent Document 3). In addition, the multiwavelength oscillation mode is set for the semiconductor laser by optical feedback using an optical fiber pigtail.
Further, a laser wavelength is shifted by Doppler shift with an acousto-optic modulator. Accordingly, a large number of different speckle patterns generate. When the speckle patterns are superimposed on one another, the speckles are averaged and reduced.
However, the spatial spectroscopy is not performed. In addition, when only a wavelength is changed, a change in phase between laser beams having a very small optical path length difference is small and speckle cannot be sufficiently removed. The spectral spread of the semiconductor laser which is caused by the multiwavelength oscillation mode is a non-uniform intensity distribution, so a spectral pattern whose intensity is large is included therein. Therefore, speckle cannot be sufficiently removed.
Patent Document 1: U.S. Pat. No. 5,634,704 B
Patent Document 2: U.S. Pat. No. 3,588,217 B
Patent Document 3: JP 2002-323675 A
Non-patent Document 1: Caesar Saloma et. al., Appl. Opt. Vol. 29, No. 6, p. 741-742